It has always been challenging to manage the flow of high temperature and high pressure solids, such as the bottom ash discharged from a fluidized bed coal gasifier, which need to be cooled and depressurized. Current systems typically use mechanical devices such as high temperature and/or high pressure valves and screws and other metering devices. However those devices are subject to severe wear and tear under the hostile environment of high temperature and pressure. Those devices also lack operational reliability. For example, a Synthetic Energy Systems® (SES) coal gasifier often uses a cooling screw, which needs to function both as a cooling device for the hot ash particles, and as a metering device to control solids flow rate. In addition to rapid deterioration of the cooling screw caused by the high temperature and high pressure environment, the cooling and metering functions of the cooling screw often are at odds with each other.
Other examples include mechanical slide valves and mechanical plug valves, which operate on fluidized flows and control the flow by modifying the cross-section of flow. There were used for fluidized bed catalytic cracking processes, operating at temperatures below 850° C., to control the circulation between various enclosures (Gauthier, IFSA 2008, Industrial Fluidization South Africa, pp. 35-87. Edited by T. Hadley and P. Smit Johannesburg: South Africa Institute of Mining and Metallurgy, 2008). These valves are particularly well-suited for operation on Group A particles of the Geldart classification (i.e. with a particle size ranging from 30-125 μm, and a density in the order of about 1,500 kg/m3). However, it is impossible to maintain Geldart Group B particles (with a particle size of about 150-1,000 μm) fluidized without forming large gas bubbles that disturb the flow. Furthermore, the moving parts of these valves cannot be exposed to temperatures higher than 900° C., making them unsuitable for bottom ash discharge in an SES gasifier.
Non-mechanical valves such as L-valves or J-valves are known and widely used in the art. An L-valve or a J-valve consists of a vertical pipe or standpipe or downcomer equipped with a horizontal pipe, or exit arm, in an angle ranging from 45-135°. If the angle is 90°, the valve adopts the appearance similar to the letter “L” and is thus called an L-valve. See e.g. U.S. Pat. No. 8,771,549 (Gauthier et al.). In an L-valve, solids are fed by gravity, or due to pressure differential, to the vertical pipe. If the vertical line is filled with particles, injection of an aeration or motive fluid, upstream from this elbow close to the change in direction, is used to promote circulation of the particles in the line. The motive fluid inlet is conventionally located at the junction opposite the exit arm, or intersecting the vertical leg proximate to the “L” junction to provide the energy to carry the solids out the exit arm. Control of the L-valve is through the control of the flow of a motive fluid to carry the solids out the exit arm. The solids flow includes fluid transported with the solids down through the vertical pipe, where the fluid in the horizontal pipe provides a motive force to facilitate all of the fluid carrying solids out the second arm, or exit of the L-valve. Generally, the solids flow rate can be controlled by adjusting the rate at which fluid is introduced at the junction.
However, L-valves have control problems due to flow instabilities for certain flow regimes. Thus, L valves are known to be only suitable for Geldart Group B particles, which have a sufficiently high minimum fluidization velocity to allow a high particle flow rate (see Geldart, D. (1973). “Types of gas Fluidization”. Powder Technology 7 (5): 285-292). A circulating bed system using L-valves to transport fine ash particles from a cyclone back into the circulating bed is described in Knowlton, T. M., (2003) “Standpipes and Non-mechanical Valves”, Handbook of Fluidization and Fluid-Particle Systems, Wen-Ching Yang, editor, pp. 571-597. Marcel Dekker, Inc., New York. U.S. Pat. No. 8,753,044 to Greenwood et al. also discloses an L-valve design for the transport of solids using a motive fluid. The valve includes an inlet conduit for carrying solids, where the solids are fed through gravity. The solids are carried by fluid transport out an outlet, where the outlet conduit has a smaller diameter than the inlet conduit. A second inlet provides the source of motive fluid to drive the transport of solids.
Previously, L-valve has not been used to handle bottom ashes discharged directly underneath an SES coal gasifier or similar fluidized bed reactors, probably because the bottom ashes contain particles that were considered to be too large (generally larger 1 mm in diameter, considered to belong to the Geldart Group D or larger, even for non-agglomerating coals) to be usable with an L-valve. L-valve aeration requirements increase with an increase in particle size, particle density, and vertical section diameter (see e.g. Knowlton, T. M.; Hirsan, I., (Conference presentation) Synthetic pipeline gas symposium, Chicago, Ill., USA, 31 Oct. 1977 Solids flow control using a non-mechanical L-valve). Operating the L-valve with Geldart Group D particles needs significantly more gas aeration compared to that with Geldart A or B particles for a given solids flow rate (see Wang and Fan, L-Valve Behavior in Circulating Fluidized Beds at High Temperatures for Group D Particles, Ind. Eng. Chem. Res., 2015, 54 (16), pp 4468-4473). For such large particles, the amount of gas needed to aerate the particles to keep them flowing in the L-valve would be very high so as to interfere with the pressure control within the fluidized bed (see Knowlton et al., supra).
There is therefore a need for an improved method and related devices that overcome the problems of the prior art.